1. Field
The present specification relates to Fresnel Incoherent Correlation Holography (FINCH).
2. Description of the Related Art
Digital coherent holography has unique advantages for many imaging applications. In some applications the recorded holograms contain three dimensional (3D) information of the observed scene, in others the holograms are capable of imaging phase objects. Holography also enables implementing super resolution techniques and even makes it possible to image objects covered by a scattering medium. Because of these advantages, digital holography has become important in optical microscopy. Examples of utilizing digital holography as the basis for optical microscopes are the recently published studies of lensless compact holography-based microscopes (T.-W. Su, S. O. Isikman, W. Bishara, D. Tseng, A. Erlinger, and A. Ozcan, “Multi-angle lensless digital holography for depth resolved imaging on a chip,” Opt. Express 18, 9690-9711 (2010); M. Lee, O. Yaglidere, and A. Ozcan, “Field-portable reflection and transmission microscopy based on lensless holography,” Biomed. Opt. Express 2, 2721-2730 (2011); O. Mudanyali, W. Bishara, and A. Ozcan, “Lensfree super-resolution holographic microscopy using wetting films on a chip,” Opt. Express 19, 17378-17389 (2011)). Another example of using digital holography in microscopy is the holographic coherent anti-Stokes Raman microscope. In the present study we extend our investigation of Fresnel Incoherent Correlation Holography (FINCH), a way to utilize holography with incoherent light, and which is another example of using digital holography in microscopy.
The setup of FINCH includes a collimation lens (objective in case of a microscope), a spatial light modulator (SLM) and a digital camera (CCD or CMOS). The principle of operation is that incoherent light emitted from each point in the object being imaged is split by a diffractive element displayed on the SLM into two beams that interfere with each other. The camera records the entire interference pattern of all the beam pairs emitted from every object point, creating a hologram. Typically three holograms, each with a different phase constant in the pattern of the diffractive element, are recoded sequentially and are superposed in order to eliminate the unnecessary parts (the bias and the twin image) from the reconstructed scene. The resulting complex-valued Fresnel hologram of the 3D scene is then reconstructed on the computer screen by the standard Fresnel back propagation algorithm (see J. W. Goodman, Introduction to Fourier optics, 3rd Ed., (Roberts and Company Publishers, 2005)). Unlike other techniques of incoherent digital holography, like scanning holography, or multiple view projection holography, FINCH is a non-scanning and motionless method of capturing holograms. Acquiring only three holograms is enough to reconstruct the entire 3D observed scene such that at every depth along the z-axis every object is in focus in its image plane. FINCH is a method of incoherent holography that can operate with a wide variety of light sources besides laser light. Because of this flexibility to practice high resolution holography with FINCH, it can be used to implement holographic applications which could not be realized in the past because they were limited by the need for coherent laser-light. Applicants have recognized additional properties of FINCH relating to resolution.
Recently two other research groups reported studies about FINCH. In one publication (Y. Tone, K. Nitta, O. Matoba, and Y. Awatsuji, “Analysis of reconstruction characteristics in fluorescence digital holography,” in Digital Holography and Three-Dimensional Imaging, OSA Techinal Digest (CD) (Optical Society of America, 2011), paper DTuC13), the authors investigated the influence of the degree of spatial coherence of light on the quality of the reconstructed 3D profiles in FINCH. In the other publication (P. Bouchal, J. Kapitan, R. Chmelik, and Z. Bouchal, “Point spread function and two-point resolution in Fresnel incoherent correlation holography,” Opt. Express 19, 15603-15620 (2011)), the authors proposed the conditions for optimal resolution with FINCH. They concluded that resolution in FINCH imaging cannot exceed that of a classical imaging system.
In this specification, the Applicants have come to different conclusions and show that indeed, FINCH imaging can exceed standard optical imaging system resolution. In the present specification, Applicants bring a more complete analysis of FINCH as an imaging system. Particularly, this specification addresses the question of which of the systems, FINCH or a conventional glass-lens-based imaging system, can resolve better. There is not an obvious answer to this question because FINCH has unique properties that do not exist in conventional optical imaging systems; on one hand, the FINCH hologram is recorded by incoherent illumination, but on the other hand this hologram is reconstructed by the Fresnel back-propagation process, exactly as is done with a typical coherent Fresnel hologram. So the question is whether FINCH behaves like a coherent or incoherent system, or whether it has its own unique behavior. Knowing that the difference between coherent and incoherent imaging systems is expressed, among others, by their different modulation transfer function (MTF), the more specific question is what kind of MTF characterizes FINCH. Does FINCH have an MTF of a coherent or incoherent imaging system, or does it have its own typical MTF? The answer to this last question can determine the answer to the resolution question. This specification analyzes the transverse resolution of FINCH and show here, both theoretically and experimentally, that FINCH imaging significantly exceeds the resolution of a conventional microscope optical imaging system.